This invention relates to detection of base line popping noise by a read head and the stabilization of the magnetic domain of that read head for a merged type magneto-resistive head used in an assembled disk drive, including GMR (Giant Magneto-Resistive) read-write heads.
Disk drives are an important data storage technology. Read-write heads are one of the crucial components of a disk drive, directly communicating with a disk surface containing the data storage medium. This invention detects base line popping and corrects this and other Electro-Static Discharge (ESD) damage to the pinned layer of the read head in an assembled disk drive. Detection reconfigures and uses thermal asperity detection circuitry included in the channel interface. Correction uses a write current applied to the write inductive coil and a read current bias applied to the read head.
FIG. 1A illustrates a typical prior art high capacity disk drive 10 including actuator arm 30 with voice coil 32, actuator axis 40, suspension or head arm 50 with slider/head unit 60 placed among the disks.
FIG. 1B illustrates a typical prior art high capacity disk drive 10 with actuator 20 including actuator arm 30 with voice coil 32, actuator axis 40, head arms 50-56 and slider/head units 60-66 with the disks removed.
Since the 1980""s, high capacity disk drives 10 have used voice coil actuators containing 20, 30, 32, 40, 50, 52, 54, 56, 60, 62, 64, and 66 to position their read-write heads over specific tracks. The heads are mounted on head sliders 60-66, which float a small distance off the disk drive surface when in operation. Often there is one head per head slider for a given disk drive surface. There are usually multiple heads in a single disk drive, but for economic reasons, usually only one voice coil actuator.
Voice coil actuators are further composed of a fixed magnet actuator 20 interacting with a time varying electromagnetic field induced by voice coil 32 to provide a lever action via actuator axis 40. The lever action acts to move head arms 50-56 positioning head slider units 60-66 over specific tracks with speed and accuracy. Actuator arms 30 are often considered to include voice coil 32, actuator axis 40, head arms 50-56 and head sliders 60-66. Note that actuator arms 30 may have as few as a single head arm 50. Note also that a single head arm 52 may connect with two head sliders 62 and 64.
Merged type heads possess different components for reading and writing, because the magneto-resistive effect only occurs during reading. A merged type head typically includes a thin film head and a spin valve sensor. The primary use of the thin film head is in the write process. The spin valve sensor is used for reading.
Merged Magneto-Resistive (MR) heads have several advantages over earlier approaches, using a single component, for both read and write. Earlier read-write heads were a study in tradeoffs. The single component, often a ferrite core, can increase read sensitivity with additional windings around the core. However, these added windings make the ferrite core write less efficiently.
Introduced in the 1990""s, merged heads brought significant increases in areal density. A merged type head reads the disk surface using a spin valve, containing a conductive thin film, whose resistance changes in the presence of a magnetic field. By separating the functions of writing and reading, each function can be optimized further than would be possible for the older read-write heads. For all the improvement that merged heads bring, there remain problems. However, before discussing these problems, consider first how and what controls these devices in contemporary disk drives.
FIG. 2A illustrates a simplified schematic of a disk drive controller 1000 controlling an analog read-write interface 220, write differential signal pair (w+and wxe2x88x92), and the read differential signal pair (r+and rxe2x88x92) communicating resistivity found in the spin valve within MR read-write head 200 of the prior art.
Analog read-write interface 220 frequently includes a channel interface 222 communicating with pre-amplifier 224. Channel interface 222 receives commands setting at least the read_bias, write_bias, and thermal asperity detection threshold(s), denoted as TA_threshold in FIG. 2A.
Various disk drive analog read-write interfaces 220 may employ either a read current bias or a read voltage bias. By way of example, the resistance of the read head is determined by measuring the voltage drop (V_rd) across the read differential signal pair (r+and rxe2x88x92) based upon the read bias current setting read_bias, using Ohm""s Law.
Most disk drives found in the prior art contain a Thermal Asperity Detection (TAD) signal as shown in FIG. 2A. The Analog read/write interface 220 generates TAD, which is sent to the embedded disk controller 1000. The embedded disk controller 1000 of the prior art contains a computer 1100 interacting 1122 with a memory 1120. The memory 1120 contains the prior art program system 1200.
FIG. 2B illustrates a suspended head slider 60 containing the MR read-write head 200 of the prior art.
FIG. 2C illustrates a perspective view of merged read-write head 200 from FIG. 2B including write inductive head 202 and magnetoresistive read head (or spin valve) 204 of the prior art.
FIG. 2D illustrates a simplified cross section view of spin valve 204 with a region 206 composed of multiple layers forming the active region of spin valve 204 of FIG. 2C of the prior art.
FIG. 2E illustrates a more detailed cross section view of region 206 of FIG. 2D, a typical GMR spin valve of the prior art.
Region 206 contains Anti-FerroMagnetic (AFM) exchange film 208 deposited on pinned Ferro-Magnetic (FM) layer 210, over a copper (Cu) spacer layer 212 in turn deposited over free layer 214 on top of under layer 216 as typically found in a GMR spin valve of the prior art.
A GMR sensor is usually fabricated as follows: AFM layer 208 primarily composed of PtMn (Platinum Manganese). Pinned FM layer 210 is primarily composed of Co (Cobalt) NiFe (permalloy). The free layer 214 is primarily composed of NiFe permalloy. Under layer 216 is often composed primarily of Tantalum (Ta).
There is a distribution blocking temperature between layers 208 and 210. When the temperature of spin valve 204 exceeds the distribution blocking temperature, the exchange coupling between AFM layer 208 and FM pinned layer 210 vanishes.
During the manufacture and handling of spin valve 204, the magnetization of pinned layer (FM layer 210) may be reversed or rotated by 180 degrees due to an ESD event. The magnetization of the free layer may also be altered by an ESD event.
Note that the entire spin valve 204 is vertically located between shields S1 and S2 of FIG. 2C as will be illustrated in FIGS. 3A and 3B.
FIG. 2F illustrates normal magnetization of a spin valve read head as well as magnetization damage from ESD events as known in the prior art.
The AFM layer 208 will typically have a magnetization direction 300. Pinned layer 210 will normally have magnetization direction 310, but after one or more ESD events, may have a magnetization direction such as indicated by 312 or 314. The Cu spacer layer 212 is not specifically relevant in this discussion and is not illustrated here. Free layer 214 normally has a magnetization direction 320 and after damage from one or more ESD events, may have an altered magnetization direction as indicated by 322.
Normally, AFM layer 208 and pinned layer 210 have essentially parallel magnetization directions and free layer 214 is magnetized essentially perpendicular to layers 208 and 210. Operation of the spin valve read head 204 depends upon these directional relationships.
FIG. 2G illustrates an even more detailed cross section view of region 206 of FIGS. 2D and 2E, a typical GMR spin valve of the prior art.
Note that layer 210 is further decomposed into an AP1 Layer, an AFC layer, and an AP2 layer.
Generally, the read head operates with a meta-stable magnetic structure. The meta-stable structure refers to a single magnetic domain at AP1, made by the strong exchange field from AFM and AP1. The meta-stable structure also refers to a single domain at free layer 214, made by the hard magnet bias. Ideally, under the meta-stable magnetic structure, free layer 214 should have stable magnetic rotation.
Magnetic single meta-stable domains naturally tend to be random, and are called xe2x80x9cmulti-magnetic domainsxe2x80x9d. Base Line Popping Noise BLPN is a name for instability in a GMR head. BLPN is most likely caused by multi-magnetic domain phenomena. The multi-magnetic domain phenomena is generally caused by electrical overstress or mechanical damage of the MR sensor. Electrical overstress is often the result of electro-static discharge. The multi-magnetic domain tends to be formed at junction and boundary regions.
FIGS. 3A and 3B illustrate the magnetic flux direction related to the charging of the write differential signal pair connecting to P1 and P2 of the prior art. P1 is related with AP1, P2 is related with AP2.
FIG. 3A illustrates the magnetic flux D1 which results from the current flowing from P1 to P2, when there is a positive write current asserted on the write differential signal pair under normal conditions in the prior art.
FIG. 3B illustrates the magnetic flux D2 which results from the current flowing from P2 to P1, when there is a negative write current asserted on the write differential signal pair under normal conditions in the prior art.
Electro-Static Discharge (ESD) can diminish or damage the pinning part of the spin valve head 204 creating a weakened or reversed magnetic condition as discussed in FIG. 2F. Such conditions damage or destroy the ability of the spin valve 204 in the MR read-write head 200 to function.
FIG. 3C illustrates a weak hard magnetic field due to edge domain problems based upon FIG. 2G leading to the phenomena of FIG. 3D as found in the prior art.
FIG. 3D illustrates the mechanism leading to base line popping due to unstable edge domain rotation as found in the prior art.
FIG. 3C illustrates a weak hard magnetic field. This allows the edge domain field to be easily moved by weak external forces. Note that the hard magnetic domain amplitude may be weak due to a large dead zone in the hard magnetic domain.
While the discussion of FIGS. 3C-3D has been made based upon edge domain effects, the same discussion applies to boundary magnetic domain effects leading to base line popping noise effects.
FIG. 4A depicts the ideal voltage amplitude measured across the read differential signal pair sensing a written pulse on a disk drive surface in the prior art.
As used in the prior art, the amplitude is defined as V++Vxe2x88x92. Asymmetry is defined as V+xe2x88x92Vxe2x88x92. The ideal situation would have a ratio of asymmetry to amplitude of 0%, but acceptable ranges are often 5% to 10%, with 7% being typical for a spin valve. ESD tends to decrease the amplitude and increase the asymmetry.
FIG. 4B illustrates base line popping noise (BLPN), a condition often adversely affecting the quality of a spin valve and resulting from certain unstable read-write heads as known in the prior art.
Base line popping can lead to false detection of peaks (1) and troughs (0) as illustrated in FIG. 4B.
Channel Statistical Measurements (CSM) are a standard system used in assembled disk drives to measure channel performance. It measures amplitude. Note that even knowing the asymmetry of a channel cannot determine the presence of base line popping noise. What is needed is a method to determine base line popping noise for specific channel conditions in an assembled disk drive.
The testing of disk drives by CSM gives only a partial quality measure. A more thorough quality measure is to determine the Bit Error Rate (BER).
The prior art teaches repairing ESD damaged and unstable read heads by raising the read head temperature above the blocking temperature and generating a magnetic field across the read head. The prior art teaches applying a high read bias current to heat the read head, often using more than 10 mA, which may melt the read head. Sometimes an external magnetic field is used, requiring an external magnet, its power supply, and mechanical infrastructure positioning the external magnet with respect to the mechanical housing of the read-write head.
The prior art approach to repairing ESD damaged and unstable read heads has both reliability and cost problems associated with it. The external magnet and its requirements add to the cost of repair and, thus, the total cost of manufacture.
FIG. 4C illustrates a thermal asperity event as found in the prior art, causing another kind of distortion to the desired waveform shown in FIG. 4A.
A thermal asperity event occurs when the read head collides with a particle on the disk surface which results in a spike in the differential read signal pairs as illustrated in FIG. 4C.
Most disk drives found in the prior art contain a Thermal Asperity Detection (TAD) signal as found in FIG. 2A previously. The prior art teaches setting TA_threshold to pass signals such as found in FIG. 4A, making the circuit useless for detecting base line popping noise as illustrated in FIG. 4B.
To summarize, what is needed is a method of detecting base line popping noise in an assembled disk drive. A method of full scale testing, within the disk drive, using the Bit Error Rate and avoiding conditions exhibiting base line popping noise, is further needed. If a disk drive is defective, an internal method of repairing the read head is needed.
The invention includes methods diagnosing and repairing read heads of merged magnetoresistive read-write heads within an assembled disk drive. The invention includes disk drives implementing such methods. The invention addresses at least the problems found in the prior art approaches.
One of the inventors realized that since base fine popping only happens when the read differential signal pair is near zero, by zeroing a track and reading that track, base line popping would be the strong effect. By setting the thermal asperity threshold small, TAD would detect the presence of base line popping when reading the zeroed track.
The invention includes determining the presence of base line popping noise for a given read head by placing the thermal asperity threshold(s) close to ground, and counting thermal asperity events while reading a zeroed track at a given read bias condition. Determining the presence of base line popping is based upon thermal asperity event counts greater than zero. Note that the invention includes combinations of thermal asperity threshold settings and TAD counts greater than some other constant than zero determining the presence of base line popping noise.
This is the only method the inventors know of which can determine the presence of base line popping noise in an assembled disk drive. This brings a new level of quality to the manufacturing of disk drives.
The invention includes determining read bias conditions free of base line popping noise in the assembled disk drive. This is done by performing the steps of determining the presence of base line popping noise for each member of a collection of read bias conditions and selecting those read bias condition members with thermal asperity event counts of zero.
The invention improves full scale testing of the read head by using the Bit Error Rate method for read bias conditions free of base line popping noise.
The invention includes repairing an assembled disk drive""s read head exhibiting base line popping. A write current source applies a write current level onto the write differential signal pair causing the write head to induce a temperature rise in the read head. A magnetic field within the read head is created by a read current source applying a read current level onto the read differential signal pair. The read current and write current are maintained for at least a time period to effect repair.
These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the drawings.